1. Technical Field:
The present invention relates generally to an improved method and apparatus for removing foreign matter, such as the products of oxidation, corrosion and sedimentation, from interior surfaces of heat exchanger vessels. The present invention has particular utility in cleaning a nuclear steam generator or other tube bundle heat exchanger by removing foreign matter accumulating on the tubesheet and on sections of the tubing adjacent the tubesheet. Other surface areas within the heat exchanger are also efficiently cleaned by the method and apparatus of the present invention.
2. Discussion of the Prior Art:
Heat exchanger-type steam generators employed in nuclear power generating systems include a primary system made up of multiple individual tubes supported on a thick metal tubesheet or base, the tubes serving as conduits for circulating primary fluid. A secondary system includes a vessel containing a secondary fluid surrounding the tubes. Thermal energy is transferred from the primary fluid in the tubes to the surrounding secondary fluid to ultimately provide the steam from which output power is derived. During operation of these steam generators there is a normal build-up of foreign matter, such as mud, sludge, tube scale and deposits of iron oxides and other chemicals, on the top surface of the tubesheet and between the closely spaced tubes. A detailed discussion of this build-up is found in U.S. Pat. Nos. 4,320,528 and 4,655,846 (both to Scharton et al). It is necessary to remove the built-up foreign material on a regular basis for a number of reasons. First, if not removed, the foreign material tends to corrode the tubes, particularly in the region of the tubesheet. Second, the foreign material interferes with the heat exchange function of the steam generator by preventing direct contact between the secondary fluid and the tubes.
In U.S. Pat. No. 3,438,811 (Harriman), a method is disclosed whereby the cleaning of internal surfaces of high pressure steam generating equipment is performed by a chemical cleaning solution. For the most part, chemical cleaning methods are less desirable than the less costly mechanical methods and generally involve a much greater risk of damage to the heat exchanger components due to chemical interaction with the tubes, etc.
Another prior art system for cleaning high pressure heat exchangers is disclosed in U.S. Pat. No. 4,320,528 (Scharton et al) and combines ultrasonic energy and a chemical solvent. Chemical cleaning is undesirable for the reason stated above. Ultrasonic cleaning has an inherent problem in that the ultrasonic energy tends to decay as it travels through the liquid medium so that the cleaning forces are strong near the transducer but relatively weak at the target areas. When cleaning a steam generator of the type described, the ultrasonic transducer must be located at the periphery of the tube bundle because there is insufficient space between tubes to position the transducer within the bundle. Consequently, high energy levels are received at the tubes near the source, tending to damage these tubes unless the applied energy is maintained relatively low. However, the low applied energy level is insufficient to effect cleaning at the center of the tubesheet and within the bundle where cleaning energy is most required. The problem, then, is how to apply sufficiently large ultrasonic energy levels to the parts requiring cleaning without damaging parts located proximate the ultrasonic energy source.
Another prior art steam generator cleaning approach is disclosed in U.S. Pat. No. 4,645,542 (Scharton et al). According to the method disclosed in this patent, repetitive explosive shock waves are introduced into the liquid-filled steam generator chamber by an air gun. The shock waves travel through the liquid and are intended to impinge upon the surfaces to be cleaned in order to loosen the products of corrosion, oxidation and sedimentation deposited and accumulated thereon. The shock wave approach, however, suffers from the same major disadvantage described above for ultrasonic cleaning, namely: space requirements demand that the pressure wave source be located outside the tube bundle, resulting in insufficient cleaning energy reaching the tubes at the bundle interior unless the source energy is so high as to risk damage to tubes located near the source.
U.S. Pat. No. 4,655,846 (Scharton et al) discloses another pressure shock wave cleaning technique. Repetitive pressure pulse shock waves are generated by an air gun, or the like, located inside or outside the chamber. The liquid in the chamber can be at a level equal to or above the support plate to be cleaned and conducts the shock waves to that plate. The liquid is continuously circulated through an external path including filters and/or ion exchange units to remove foreign materials loosened by the shock waves. Again, the use of shock waves at sufficient pressure to clean interior components carries the risk of damage to components located proximate the shock wave source.
The water-slap method disclosed in U.S. Pat. No. 4,756,770 (Weems et al) effects cleaning by repetitive impacts against the surface to be cleaned by a rapidly rising surface of a pool of liquid disposed in the steam generator chamber. Surfaces cleaned in this manner include horizontal support plates and nearby tube sections. The surfaces to be cleaned must initially be located at least a few inches above the surface of the pool of liquid so that the pool can be accelerated upwardly and create the necessary impact. One technique for achieving the desired upward acceleration of the liquid is repetitive injection of nitrogen gas deep within the pool to form a bubble that drives the pool upwardly. The liquid is typically water and is continuously circulated through an external path wherein solid particles are removed. It is impossible to clean the top surface of the tubesheet and adjacent tube sections with the water slap method. Specifically, the top surface of the tubesheet constitutes the bottom of the chamber in which the water pool sits, thereby precluding locating the pool surface a few inches away from the tube sheet top surface as would be required by the water slap method to achieve the intended acceleration and impact. On the other hand, it is the very location of the tubesheet at the bottom of the chamber that causes foreign matter to accumulate thereon, and on adjacent tube sections, so as to require frequent cleaning.
Another known method for cleaning steam generators, disclosed in U.S. Pat. No. 4,079,701 (Hickman et al), is called sludge lancing wherein cleaning is effected by flow impingement and hydraulic drag forces. The components to be cleaned by this process, namely support plates, tubesheets and possibly tubes, are not submerged. Rather, a nozzle directs liquid (e.g., water) jets to impinge upon the areas to be cleaned. Only small localized areas can be cleaned at any one time, and the nozzles must be moved about within the heat exchanger to clean all of the desired surfaces. In order to provide access to these surfaces, it is necessary to cut a relatively large number of access holes in the pressure retaining shell of the heat exchanger so that nozzles and tubing can be appropriately oriented. These holes must be plugged or otherwise sealed after the cleaning process. The cutting and plugging requirement adds significantly to the overall cost of the cleaning process.